High pressure electric discharge device with zirconium-aluminum getter

ABSTRACT

Strips of zirconium-aluminum are disposed within a high pressure metal iodide discharge lamp to getter hydrogen which is entrapped therein. The strips are welded to support straps on each of the arc tube press seals to thereby dispose the getter at each end of the lamp at locations where the operating temperature will be maintained at about 500*C.

United States Patent 1191 Gungle [54] HIGH PRESSURE ELECTRIC DISCHARGE DEVICE WITH ZIRCONIUM-ALUMINUM GETTER [75] Inventor:

[73] Assignee: GTE SyIvania'Incorporated,

Danvers, Mass.

221 Filed: June so, 1971 21 Appl.No.: 158,422

Warren C. Gungle, Danvers, Mass.

[52] US. Cl 313/25, 313/174, 313/184 [51] Int. Cl H01j 61/26, HOlj 61/52 [58] Field of Search 313/184, 25, 174, 227;

[56] References Cited UNITED STATES PATENTS 3,450,925 6/1969 Johnson 313/184 X 51 Apr. 16, 1974 2,749,462 6/1956 Kenty et a1. 313/25 3,626,229 12/1971 Spacil et al. 313/184 X 3,203,901 8/1965 Della Porta 252/l8l.6 3,620,645 11/1971 Della Porta et al... 252/l8l.6 X 2,948,607 8/1960 Wagener 313/174 Primary Examiner-Palmer C. Demeo Attorney, Agent, or Firm-Edward J. Coleman [5 7] ABSTRACT Strips of zirconium-aluminum are disposed within a high pressure metal iodide discharge lamp to getter hydrogen which is entrapped therein. The strips are welded to support straps on each of the arc tube press seals to thereby dispose the getter at each end of the lamp at locations where the operating temperature will be maintained at about 500C.

5 Claims, 2 Drawing Figures PATENTEDAPR 16 I974 WARREN C. GUNGLE I VENTOR B M ATTOR Y HIGH PRESSURE ELECTRIC DISCHARGE DEVICE WITH ZIRCONIUM-ALUMINUM GETTER BACKGROUND OF THE INVENTION '1. Field of the Invention This invention relates to high pressure electric discharge devices and particularly to those which utilize fills of mercury or mercury and halogens. Such devices generally include a quartz arc tube which contains the fill and is supported upon a wire frame which is disposed within an outer bulbous envelope.

2. Description of the Prior Art High pressure electric discharge devices containing mercury and also those containing mercury and halogens are known to the art. The art has recognized that hydrogen, which can be a contaminant in these devices, is detrimental to the operation. When trapped within the bulbous envelope, the hydrogen diffuses through the quartz wall of the arc tube and adversely affects both starting and reignition voltages. The hydrogen migrates into the arc tube and forms in the case of iodine fills, hydrogen iodide-which is a volatile iodine containing species and exists as a gas at temperatures even as low as 20F. At low ambient temperatures, the effect of hydrogen contamination is especially noticeable because the presence of the corresponding iodide produces high starting voltages. Moreover, the presence of hydrogen iodide in the arc tube results in a high value of voltage required to reignite the lamp each half cycle of alternating current during the warm-up phase of the lamp operation. This voltage, referred to hereafter as reignition voltage, is an important parameter in determining whether a lamp can operate reliably on a given ballast circuit. The lower it is, the more reliable will be operation, or conversely the more economical will be the ballast design to reach a desired level of reliability.

We have discovered that one of the sources of hydrogen in such devices is the bulbous envelope. Ultraviolet light emitted from the arc tube releases hydrogen from hydroxyl radicals which are entrapped in the glass jacket.

Getters, that is materials which entrap extraneous gases, have previously been utilized in such devices. Gettering, as usually practiced in the art, involves flashing or volatilizing barium metal to react with gases, thereby removing them from the system. However, such procedures not only remove the hydrogen, but also getter nitrogen which is intentionally added. Since a gas should be present within the envelope, replacement of the nitrogen with argon would be required since this gas is not gettered. But because the use of argon reduces the potential where arcing between elements of the lamp can occur, it is not as satisfactory as nitrogen. Thus, the use of conventional barium getters has serious disadvantages. The same is true of the socalled flashless getters, such as tantalum, cerium, or alloys containing these metals, such as are known to the art. All of these react rapidly with nitrogen as well as hydrogen and would require replacement of the nitrogen fill gas of the outer jacket by argon.

A method of removing hydrogen from the outer jacket, without appreciably affecting the nitrogen content, is described by U.S. Pat. No. 3,519,864, issued July 7, 1970 and assigned to the assignee of the present application. This patent employs barium peroxide as the getter and disposes the material at a location in the outer jacket where the temperature is normally expected to lie between 159 and 427C. Within this temperature range, barium peroxide effectively getters hydrogen without producing deleterious by-products and without significantly reacting with the nitrogen fill gas. As barium peroxide decomposes when irradiated with ultraviolet light, the getter material is held in a container opaque to ultraviolet light, such as a pair of foraminous plates. The getter container is mounted at the end of the bulbous envelope away from the base, where the temperatures are expected to lie within the aforementioned range.

Although the barium peroxide getter has been extremely advantageous inmaintaining low hydrogen vapor pressure, and as a consequence low reignition voltage for lamps, the above described position of the getter package is far enough removed from the arc tube that the ambient temperature about the lamp significantly influences the getter temperature. In the case of metal iodide lamp applications, the ambient tempera ture may vary by as much as 200C or more. The lowest temperatures would occur in open fixtures out of doors in the winter time, whereas high ambients of upwards of 400C or higher occur in small vapor tight fixtures. At low temperatures, i.e. below C, the rate of hydrogen removal by the barium peroxide is too low to provide effective gettering. On the other hand, if the getter package reaches temperatures above 427C, as can occur at the higher ambient temperatures, the barium peroxide will decompose and yield barium oxide and free oxygen. Accordingly, the ambient temperature sensitivity of the barium peroxide package, due to its location in the outer jacket as dictated by its working temperature range, makes this type of getter unsuitable for discharge device applications subject to ambient temperature extremes.

Similar disadvantages apply to a mercury vapor lamp described in US. Pat. No. 2,749,462 which employs a strip of zirconium to provide selective hydrogen gettering. The lamp contains an arc tube having tapered ends which fit within perforations in respective flexible metal supports having a lateral, thin, plate-like configuration. A inch strip of the zirconium getter material is welded to the top metal support of the arc tube in such a position that one of its sides faces the tapered end of the arc tube. At this location of the getter, the normal operating temperature is about 400.

SUMMARY OF THE INVENTION In view of the aforementioned shortcomings of the prior art, it is an object of this invention to provide an improved high pressure electric discharge device which is comparatively insensitive to ambient temperature variations with respect to its capability of maintaining relatively low reignition voltages.

A more particular object of the invention is to provide a metal iodide lamp having an improved getter arrangement for maintaining relatively low hydrogen levels in the outer jacket without significantly affecting nitrogen content and providing optimum independence of lamp ambient temperature.

A further object is to provide a metal iodide lamp having an improved getter arrangement which provides greater application flexibility by rendering the lamp suitable for use in a wide variety of fixtures and mounting orientations.

Briefly, these and other objects are attained by welding strips of zirconium-aluminum getter material to the faces of press seal support straps at each end of the arc tube. Such an arrangement optimizes the temperature stability of the getter location and provides gettering action substantially independent of the type of lamp fixture, the mounting attitude, and variations in the ambient temperature about the lamp. The operating temperature at each of the getter locations is approximately 500C, which is within the range where the gettering rate for hydrogen peaks.

BRIEF DESCRIPTION OF THE DRAWINGS The invention will be more fully described in the following detailed description in conjunction with the accompanying drawings, in which:

FIG. 1 is an elevational perspective view of a high pressure metal iodide discharge lamp having a getter arrangment in accordance with the invention; and

FIG. 2 is an enlarged detail view of the top portion of the arc tube in FIG. 1 showing a strip of getter material mounted in accordance with the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT Referring to FIG. 1 of the drawing, the lamp includes a generally tubular outer bulbous envelope 1 having a bulbous central portion and a conventional base 14 attached to the bottom thereof. Extending inwardly from the base and inside of the envelope 1 is a mount 15 having a pair of stiff lead-in wires 12 and 16 in electrical conducting relation with the base 14. Disposed upon one of the stiff lead-in wires 12 is a lower U-shaped support 8 welded thereto. The U-shaped support 8 comprises a pair of vertical wires 23 and 24 rising from a horizontal base wire 25. The upper ends of the lower U-shaped support 8 are welded together with a lower metal strap 7 which in turn supports a quartz glass arc tube 2. Preferably, the lower metal strap includes two sections abutting against either side of the arc tube 2 thereby holding it firmly in place and touching only the press seal 38 of the arc tube and not the body. Generally, both sides of the lower strap 7 can be of identical construction. A pair of bumpers 26 are welded to the lower U-shaped support 8 and abut against the tubular portion of walls of the outer-bulbous envelope 1, thereby stabilizing the structure within the lamp. Preferably, these bumpers are made of a resilient material so that if the lamp is jarred, they will absorb much of the shock.

Since the lower U-shaped support 8 is electrically connected to the stiff lead-in wire 12, the support 8 forms part of the circuit in the device. Current passes from the base 14 into the lower U-shaped support 8 and thence to lead-in wire 21 which in turn is connected to an electrode 4 in the arc tube. It is sometimes desirable to place an insulating shield about the lead-in wire 21 to prevent arcing within the lamp and between the various elements. Current passes from the lead-in wire 21 to the electrode 4 through an intermediary molybdenum foil section 6.

The other side of the circuit is formed through the stiff lead-in wire 16 which is preferably bent out of place so that the parts on one side of the line are insulated from those on the other side. A resistor 13 is attached to the stiff lead-in wire associated therewith and thence to a connector 27 which in turn leads through a molybdenum foil section 6 to a starting probe 5. A bimetal 22 is disposed between lead-in 21 and the lead-in wire 29 which is attached to the electrode 4. The bimetal 22 is biased open when the device is turned off, but upon starting it biases closed against the lead-in wires to the probe 5, thereby establishing the same current potential at the probe 5 and the electrode 4. Such closing prevents electrolysis between the probe and electrode.

At the other end of the arc tube 2, an upper support 10 is mounted within the tubular portion of the bulbous envelope 1. The support frame 10 includes a horizontal section 18 having vertical supports 17 and 19 depending downwardly therefrom and attached at the free ends to an upper metal strap lll which surrounds the press seal 40 of arc tube 2 and rigidly holds it in place, as shown in FIG. 2. Preferably, the construction and disposition of upper metal strap 11 is similar to lower strap 7. A pair of upper bumpers 9 are mounted upon the vertical sections 17 and 19 0f the upper support 10 and resiliently abut against the sides of the tubular portion of the bulbous envelope 1. Such disposition prevents breakage of the lamp if the arc is shaken or dropped.

A lead-in wire 28 extends to the outside of the arc tube 2 and is attached at its inner end to a molybdenum foil section 6 and thence to an electrode 3. An electrical connection is made between stiff lead-in wire 16 and lead-in wire 28 through a thin conducting lead 20 which may be of any suitable conducting material. Preferably, the conducting lead 20 is as distantly removed from the arc tube 2 as possible, generally by bending it around the perimeter of the outer bulbous envelope 1.

The envelope 1 of the lamp is filled with nitrogen at a pressure of about half an atmosphere at room temperature to minimize the occureence of arc-overs between the electrical conductors therein, beneficially effect the temperature distribution over the arc tube 2, and reduce photoelectric current flowing to the arc tube.

In accordance with the invention, a strip 34 of zirconium-aluminum getter material measuring 3.2mm X 4.5mm, with a thickness of 0.01 1 of an inch, and weighing about 250 milligrams is attached, preferably, by welding, to one face of the upper metal strap 11, as illustrated in FIG. 2. One type of zirconium aluminum getter suitable for this application is STlOl, available from SAES Getters Inc. of Milan, Italy. By attaching the face of getter strip 34 to the face of metal strap 11, which in turn is securely abuting the press seal 40, a path of optimum heat transfer is provided between the arc tube heat source and the getter strip. As a result of this optimized thermal proximity of the getter to the arc tube heat source, and its relative remoteness from the outer jacket (envelope 1), the stability of the getter temperature is optimized. More specifically, l have discovered that the lamp construction of FIGS. 1 and 2 provides a getter-ing action which is substantially independent of the ambient temperature about the lamp.

In a preferred embodiment, a second strip 36 of zirconium-aluminum getter is similarly welded to the lower strap' 7 on its oppositely disposed face, as illustrated in FIG. 1. By attaching getter strips to both ends of the arc tube in this manner, no need exists for differentiating between attitudes of operation.

The operating temperature on each of the strap faces is approximately 500C. As the zirconium-aluminum gettering rate for hydrogen peaks in the region of 400-500C, this arrangement of getter strips 34 and 36 assures rapid removal of residual hydrogen gas from the outer jacket, between arc tube 2 and envelope 1, without the possibility of introducing oxygen. I have further discovered that the gettering of hydrogen gas at 500C is selective; that is, at this temperature, the zirconium-aluminum getter has no significant reaction with nitrogen. In fact, at 500C, the getter will maintain a hydrogen equilibrium pressure of Torr or lower. As the outer jacket hydrogen level is in equilibrium with that within the arc tube under operating conditions, the maintenance of a low outer jacket hydrogen level will in turn maintain a low arc tube hydrogen level, thereby precluding an undesired increase in the reignition voltage. For example, I have found that reignition voltages of 1,000 watt metal iodide lamps are reduced to 60V on lamps which have been aged for 1,000 hours. The reignition voltage of the non-gettered control lamp was 100V. Moreover, the performance after 1,000 hours is substantially better than the zero hour or initial performance, thereby indicating that the getter has even removed hydrogen which was originally present inside of the arc tube 2; this hydrogen has apparently diffused outward through the quartz wall of the operating arc tube into the outer jacket where the getter could react with it.

In summary, the temperature stabilized getter arrangement of the present invention provides a high pressure discharge lamp which has significantly greater application flexibility than prior art types. In particular, it provides high performance operation which is far less sensitive to the variations in ambient temperature and mounting orientation encountered with different operating fixtures.

What I claim is:

1. A high pressure electric discharge device comprising: a sealed bulbous glass envelope; a quartz glass are tube disposed within said sealed bulbous envelope, said are tube having press seals at each end; means to form an electric discharge within said arc tube; means supporting said are tube within said envelope including metal straps abutting said press seals at each end of said arc tube; and a hydrogen getter consisting of a strip of zirconium-aluminum metal attached to the face of one of said metal straps at one end of said arc tube, with substantially an entire face of said getter strip being juxtaposed to the face of said metal strap, whereby the temperature of said getter during lamp operation is stabilized to be substantially independent of the ambient temperature about said lamp.

2. A device according to claim 1 wherein said getter strip is attached by welding to the face of one of said metal straps.

3. A device according to claim 1 wherein the thermal path from said are tube to said getter strip provides for the heating of said getter strip to a temperature of about 500C. during operation of said lamp.

4. A device according to claim 1 wherein a second strip of zirconium-aluminum getter is attached to the face of one of said metal straps at the other end of said arc tube, with a face of said second getter strip being juxtaposed to the face of said metal strap.

5. A device according to claim 4 wherein said device is a metal iodide lamp, the thermal paths from said arc tube to said getter strips provide for the heating of each of said getter strips to a temperature of about 500C during operation of said lamp. 

2. A device according to claim 1 wherein said getter strip is attached by welding to the face of one of said metal straps.
 3. A device according to claim 1 wherein the thermal path from said arc tube to said getter strip provides for the heating of said getter strip to a temperature of about 500*C. during operation of said lamp.
 4. A device according to claim 1 wherein a second strip of zirconium-aluminum getter is attached to the face of one of said metal straps at the other end of said arc tube, with a face of said second getter strip being juxtaposed to the face of said metal strap.
 5. A device according to claim 4 wherein said device is a metal iodide lamp, the thermal paths from said arc tube to said getter strips provide for the heating of each of said getter strips to a temperature of about 500*C during operation of said lamp. 